1. Field of the Invention
This invention relates to a method of determining a cutting trajectory of a tool when machining a work by an N/C (numerical control) machining system, for instance, on the basis of three-dimensional shape data prepared by CAD (computer-aided design).
2. Description of the Prior Art
Recently there has been put into wide use an automatic programming system in which a computer is employed to control an NC machining system.
For example, when a pressing die having a complicate curved surface is machined, the surface of a work is directly cut using N/C data prepared from CAD (computer-aided design) data without using a plaster master or the like.
In the N/C machining system, the tool for cutting a work is first positioned away from the work and is moved toward the work to be brought into contact with the work at a part to be cut. After cutting the part, the tool is removed from the work, and is moved to another part to be cut if any, and finally is returned to the original position. The path along which the tool is moved during cutting operation on each part to be cut is determined on the basis of the shape of the surface to be formed thereon using the CAD data. In this specification, the path of the tool along which the tool is to be moved during cutting operation on each part to be cut will be referred to as "cutting trajectory", and the path of the tool along which the tool is to be moved in the whole cutting operation including the path aloha which the tool is moved toward the work from the original position to the inlet end of the cutting trajectory for the first part to be cut, the path along which the tool is returned to the original position from the outlet end of the cutting trajectory for the last part to be cut and the paths along which the tool is moved from part to part as well as the respective cutting trajectories will be referred to as "tool running path" in order to make a distinction therebetween.
When cutting a work with a ball end mill, a region at a corner portion of the work is left uncut as can be seen from From FIG. 11 when the diameter of the ball end mill R is large. (The region left uncut will be referred to as "leftover portion" in this specification.) The leftover portion must be manually finished later, which results in deterioration in accuracy and increase in man-hours. On the other hand, when the work is cut with a ball end mill having such a small diameter that can cut the corner portion with no leftover portion, the tool feed speed must be greatly lowered, which results in great increase in the machining time.
In view of the problems described above, the leftover portion produced by a large diameter ball end mill is cut with a plurality of tools which are gradually reduced in diameter. See, for instance, Japanese Unexamined Patent Publication No. 3(1991)-62204. In order to execute this method, the cutting trajectories for the small diameter ball end mills must be prepared in addition to those for the large diameter ball end mill.
When a work is cut with a plurality of ball end mills having different diameters, conventionally, the cutting trajectory is once prepared also for the small diameter (r) ball end mill over the entire cutting area of the work as shown in FIG. 12. Then the leftover portion is determined reading the drawing and is represented by broken lines which are approximated to the shape of the curved surface to be formed as shown in FIG. 13A. Then the part of the cutting trajectory other than the part for the leftover portion is trimmed and the cutting trajectory for cutting the leftover portion with the small diameter ball end mill in the same direction as the direction of the cutting trajectory for the large diameter ball end mill is output from the computer as shown in FIG. 13B.
In accordance with the conventional method described above, a very long time is required to calculate the cutting trajectory data by the computer and due to a large amount of data, a large capacity memory is required.
Further, determination of the leftover portion, representation of the leftover portion by the broken lines and trimming of the cutting trajectory which are executed by the operator take a long time and at the same time the accuracy of the data deteriorates due to such manually-performed editing operations.
Further since the leftover portion is formed as an elongated area extending perpendicular to the cutting trajectory for the large diameter ball end mill, the small diameter ball end mill must be moved back and forth many times over a very short distance, which greatly adds to the machining time.